The use of spray-drying to produce powders from fluid feed stocks is well known, with applications ranging from powdered milk to bulk chemicals and pharmaceuticals. See U.S. Pat. No. 4,187,617 and Mujumbar et al., Drying 91, pages 56-73 (1991). The use of spray-drying to form solid amorphous dispersions of drugs and concentration-enhancing polymers is also known. See commonly owned European Patent Applications Nos. 0 901 786, 1 027 886, 1 027 887, 1 027 888, and commonly-owned PCT Applications Nos. WO 00/168092 and WO 00/168055. And the use of a perforated plate as an air disperser for a spray-dryer using a nozzle atomizer is also known. See Masters, Spray Drying Handbook, pages 263-268(4th ed 1985).
A typical spray-drying apparatus comprises a drying chamber, atomizing means for atomizing a solvent-containing feed into the drying chamber, a source of heated drying gas that flows into the drying chamber to remove solvent from the atomized solvent-containing feed and product collection means located downstream of the drying chamber. Examples of such apparatus include Niro Models PSD-1, PSD-2 and PSD-4 (Niro A/S, Soeborg, Denmark). When used for forming solid amorphous dispersions by spray-drying, conventional wisdom suggests that to achieve rapid removal of solvent required to form a homogeneous solid amorphous dispersion, the droplets of atomized solvent-containing feed should be small. The prior art therefore uses spray-drying apparatus equipped with a two-fluid nozzle for atomizing the solvent-containing feed. Such nozzles produce small droplets of feed solution, typically 5 to 30 μm in diameter, and turbulent mixing of the liquid feed droplets and drying gas, leading to rapid drying of the fluid to form solid particles. When used in the prescribed manner, such spray-drying apparatus are effective in forming substantially amorphous and substantially homogeneous solid-amorphous dispersions of drug and polymer that show concentration enhancement when introduced to an environment of use. However, as noted above, the spray-dried particles produced in such apparatus often have small median particle sizes (less than about 30 μm in diameter) and a large amount of “fines” (particles with diameters of less than about 10 μm). The product also typically has a high specific volume. Specific volume is the volume of the spray-dried powder divided by its mass—typically reported in units of cm3/g. Generally, the higher the specific volume of a powder, the poorer its flow characteristics. As a result, the dispersions produced using a spray-drying apparatus equipped with a two-fluid nozzle have relatively poor flow characteristics and poor collection efficiency.
The inventors have found that the flow characteristics and collection efficiency of spray-dried dispersions can be improved by using a spray-drying apparatus equipped with atomizing means that produces droplets with an average droplet diameter of 50 μm or larger, with less than about 10 vol % of the droplets having a size less than 10 μm. Such an atomizing means is referred to herein as a “pressure nozzle.” It has been discovered that homogeneous solid amorphous dispersions formed using pressure nozzles have relatively larger median particle sizes, with minimal fines present. The resulting dispersions therefore have improved flow characteristics and improved collection efficiencies. See commonly owned U.S. Provisional Application No. 60/353,986 filed Feb. 1, 2002 and incorporated herein by reference.
However, all else being equal, the rate of removal of solvent from such larger droplets produced by a pressure nozzle is slower than that from smaller droplets, such as those produced by a two-fluid nozzle. Conventionally, to counteract this tendency for large droplets to dry more slowly, drying gas is introduced in a flow direction that is not parallel to the atomized droplet flow. Drying gas introduced in this manner induces large circulation cells that carry droplets or particles initially directed downward back up to the top of the dryer. Such flow causes turbulent mixing of the drying gas and atomized spray solution, leading to more rapid drying of the droplets. However, these conventional methods for spray-drying large particles result in (1) build-up of material on the nozzle itself, as well as on the dryer surface near the drying gas inlet, (2) excessively rapid drying of some of the particles, and (3) less uniform drying conditions. As a result, the product produced tends to have poor content uniformity, high specific volumes, poor flow characteristics, and when the build-up occurs on hot surfaces, the potential for chemical degradation of the product. Thus, such non-parallel introduction of drying gas to a conventional spray-drying apparatus should be avoided.
There is therefore a need in the art for an improved spray-drying process that results in the production of solid amorphous dispersions at high yield with improved flow characteristics, improved content uniformity, and improved collection efficiency.